The present invention relates to surface monitoring, and in particular, to apparatus for monitoring certain distribution characteristics of droplets distributed over a surface. For the purpose of explanation, a preferred embodiment of the invention is described herein in conjunction with the manufacture of plywood where it has been found to have particular utility.
It is desirable, in the manufacture of ply articles such as plywood, to monitor and control the weight of glue sprayed onto the individual ply or veneer. One approach which may be used to monitor applied glue-weight is to determine the size and density distribution characteristics of the droplets on the veneer, where these characteristics are related in a known way to the weight of glue applied. It is advantageous, in such monitoring, to be able to distinguish and reject from the glue-weight calculation, surface features such as holes, knots and gaps between adjacent veneer pieces.
A variety of devices used to monitor surface characteristics in traveling workpieces are known in the prior art. Typically in such devices, the angle of light reflected from the surface of a moving workpiece is determined to provide a measure of surface roughness or other irregularities. Devices such as these are generally ineffective in monitoring droplet-like surface features which tend to have relatively flat surfaces and therefore reflect light at an angle which is indistinguishable from that of uncovered surface regions.
Accordingly, one general object of the present invention is to provide apparatus for monitoring the size and density distribution of glue droplets or the like applied to a traveling workpiece.
Another object of the invention is to provide such an apparatus which is capable of distinguishing droplets on a workpiece surface from relatively large features such as surface holes, knots and gaps between adjacent workpieces.
Still another object of the invention is to provide such an apparatus which is capable of monitoring the performance of a glue applicator, and of controlling the glue application rate thereof.
The apparatus of the present invention includes a light source for directing a beam of light onto the surface of a traveling workpiece, where the surface of the workpiece has been sprayed with droplets whose light reflectance differs in a known way from that of uncovered regions of the surface. The level of light reflected from the traveling surface is detected by a photodetector which produces a signal that varies according to the instantaneously detected reflected light level. The signal is transformed to a related binary data stream composed of one and other voltage states representing reflected light levels from covered and uncovered regions, respectively, on the surface. Droplet distribution characteristics related to the weight of glue applied to the workpiece surface are computed from the data stream.
In a preferred embodiment of the invention, the apparatus of the invention includes a bit gate filter for ignoring, in the data stream, sequences having a predetermined number of successive, identical data states. This filter enables the apparatus to distinguish droplets from large non-droplet surface features such as those mentioned above.